Ondansetron (Zofran) is a serotonin 5-HT3 receptor antagonist used to treat nausea/vomiting. It has also been found to potentially be helpful in treating OCD.

I actually have been taking Ondansetron myself for several weeks now, and I honestly can tell there is a slight reduction in my OCD symptoms, primarily my obsessive thoughts. (I didn't even know it had the potential to ease my OCD symptoms until later after having been on it for a while, to treat nausea, so not placebo.)

It seems to slightly calm my brain down, as in, less focus on my obsessive thoughts when on it.